This invention relates to an outboard motor and more particularly to an exhaust device for use in a four-cycle outboard motor.
Outboard motors are normally powered by two-cycle engines because of the simplicity of these engines. However, there are many instances when it is desirable to employ a four-cycle internal combustion engine as the propulsion unit for an outboard motor. If a four-cycle engine is employed, it is necessary to provide an oil sump for the lubrication system of the engine. The oil sump may be located within the engine casing in the form of an oil pan.
It is further known, with many outboard motors, to cool an engine with liquid obtained from the body of water in which an associated watercraft is operated. The water is circulated through the cylinder block and cylinder head of the engine to cool the engine. In addition, the exhaust gases are discharged downwardly through at least a portion of the drive shaft housing for expulsion either into the body of water or directly into the atmosphere. It is often desirable to provide cooling for the exhaust gases before they are discharged so as to avoid unnecessary overheating of the drive shaft housing and to serve a silencing function during engine operation. One method which has been proposed for cooling the exhaust gases is to surround an exhaust gas flow passage with cooling water. Another alternative is to discharge the coolant into an exhaust expansion chamber.
As has been noted, the coolant for a water-cooled outboard motor may be drawn from the body of water in which the motor operates. Frequently, the motors are operated in salt water. When a four-cycle outboard motor is constructed so that the coolant is introduced between its oil pan and its exhaust passage, or into its exhaust expansion chamber, the coolant may frequently come into contact with, and adhere to, the outer surface of the oil pan. If salt water is used, the salt can be crystallized by the heat of the exhaust gases. As the crystallization proceeds, the crystallized salt absorbs moisture from the air and corrodes the outer surface of the oil pan with oxygen obtained therefrom.
In addition, in a conventional arrangement for a four-cycle outboard motor, hot exhaust gases released into an expansion chamber may contact the outer surface of the oil pan, thereby imparting heat to the lubricant contained therein. Such undesirable heating of the lubricant can cause it to deteriorate, rendering the lubricant much less effective.
It is, therefore, a principal object of this invention to provide an improved exhaust gas cooling arrangement for a four-cycle outboard motor.
It is a further object of this invention to provide an exhaust gas device which can prevent salt from seawater coolant from adhering to and corroding the outer surface of an oil pan.
It is yet another object of this invention to prevent the undesirable breakdown of engine lubricant due to its contact with hot exhaust gases.
It is still a further object of this invention to provide a liquid cooling arrangement for an engine effective to maintain the lubricating and exhaust components of an outboard motor at a relatively low and efficient temperature during engine operation.